Method and device for measuring gamma radiation

Abstract

A method and device for measuring peak gamma radiation is provided wherein the method includes the steps of determining a minimum and maximum number of counts within a preselected multiple channel energy spectrum measurement range, determining a channel having approximately one-half the number of maximum counts, determining a channel representing a background energy, computing a peak energy centroid channel, computing a channel to energy conversion factor and recomputing the boundry channels for the preselected multiple channel energy spectrum measurement range. An apparatus for measuring gamma radiation is also provided that includes a device for detecting gamma radiation emmissions and producing an electrical signal of a magnitude proportional to the level of energy of the emmision connected to a converter that transmits a digital word containing the emission energy level to a computer circuit that computes the number of emissions for specific energy level channels within a first set of channel boundaries and then calibrating new channel boundaries as a function of the number of emissions for the specific energy level channels within the first set of boundaries.

What is claimed is: 
     
       1. A method of measuring gamma radiation comprising: (a) determining a minimum and a maximum number of counts within a preselected multiple channel energy spectrum measurement range having a lower boundary channel corresponding to a selected minimum energy value and an upper boundary channel corresponding to a selected maximum energy value;   (b) determining a channel having approximately one-half of the maximum number of counts;   (c) determining a channel representing Compton background energy;   (d) computing a peak energy centroid channel from the relationship between the channel having one-half of the maximum number of counts and the channel representing the Compton background energy;   (e) computing a channel to energy conversion factor; and   (f) computing calibrated lower and upper boundary channels, respectively, by multiplying the channel to energy conversion factor by the selected minimum energy value and the selected maximum energy value   
     
     
       2. The method of claim 1 wherein said step for computing the energy to channel conversion factor includes the step of dividing the peak energy centroid channel by a known spectral peak energy constant. 
     
     
       3. A method according to claim 2 wherein the step for computing the peak energy centroid channel includes the step of dividing the channel having one-half of the maximum number of counts by a quantity including a constant times the square root of the natural logarithm of the ratio of the maximum number of counts minus the Compton background divided by the counts within the channel having approximately one-half the maximum number of counts adjusted by the Compton background. 
     
     
       4. A method according to claim 3 wherein the channel having approximately one-half of the maximum number of counts is determined by multiplying the channel with the maximum number of counts times a known constant representing a resolution factor. 
     
     
       5. A method according to claim 4 wherein said step of determining the channel representing the Compton background energy includes the multiplying of the channel having the maximum number of counts by a second constant representing a resolution factor. 
     
     
       6. A method for calibrating a device for measuring gamma radiation within a multiple channel energy spectrum measurement range having lower and upper channels corresponding, respectively, to minimum and maximum energy values of the energy spectrum, said method comprising: (a) determining a minimum count channel and a maximum count channel within the range;   (b) determining whether a peak has been detected within the range by the relationship between the counts of the minimum and maximum count channels;   (c) upon determining that a peak has been detected, determining a first channel having approximately one-half of the maximum number of counts;   (d) determining a second channel representing Compton background energy;   (e) computing a peak energy centroid channel from the first channel and the second channel;   (f) computing a channel to energy conversion factor from the peak energy centroid channel; and   (g) computing lower and upper calibrated measurement range channels respectively by multiplying the energy to channel conversion factor by the minimum energy value and the maximum energy value   
     
     
       7. A method according to claim 6 further including steps: (h) determining a minimum and a maximum number of counts within the lower and upper calibrated measurement range channels; and   (i) repeating steps (b) through (g)   
     
     
       8. A method according to claim 7 wherein steps (b) through (i) are repeated periodically for calibrating the measuring device. 
     
     
       9. A method according to claim 8 wherein said energy spectrum measurement range includes a measurement range for potassium having a minimum energy value of approximately 1.37 MeV and maximum energy value approximately 1.57 MeV. 
     
     
       10. A method according to claim 9 wherein the energy to channel conversion factor is used to determine second and third calibrated measurement range channels for the measurement of gamma radiation from uranium and thorium. 
     
     
       11. A method according to claim 8 wherein said energy spectrum measurement range includes a measurement range for thorium having a minimum energy value of approximately 2.30 MeV and a maximum energy value of approximately 2.80 MeV. 
     
     
       12. A method according to claim 11 wherein the energy to channel conversion factor is used to determine second and third calibrated measurement range channels for the measurement of gamma radiation from potassium and uranium. 
     
     
       13. An apparatus for measuring gamma radiation comprising: means for detecting gamma radiation emissions and for producing electrical signal each of said signals being of a magnitude proportional to a level of energy of each emission;   conversion means for converting the electrical signals produced by the detecting means to digital words each of said words corresponding to a particular emission energy level;   means for counting the number of words corresponding to each emission energy level within a gamma ray stripping window having a lower boundary word corresponding to a lower boundary energy level and an upper boundary word corresponding to an upper boundary energy level;   means for computing a word to energy conversion factor;   means for computing a stabilized lower boundary word by multiplying the lower boundary energy level by the word to energy conversion factor; and   means for computing a stabilized upper boundary word by multiplying the upper boundary energy level by the word to energy conversion factor.   
     
     
       14. An apparatus for measuring gamma radiation according to claim 13 further including means for iteratively determining new calibrated upper and lower boundary words as a function of the number of emissions for the specific energy level words within the previous boundaries. 
     
     
       15. A device according to claim 14 wherein said detection means includes a sodium activated cesium lodide crystal connected to a photomultipler tube wherein said cesium iodide crystal and said photomultipler tube are located within a vacuum flask. 
     
     
       16. A method of stabilizing a gamma ray stripping window in a gamma ray energy spectrum, said stripping window having a lower boundary energy value, an upper boundary energy value, and a peak energy value positioned between said lower and upper boundary energy values, which comprises the steps of: partitioning said gamma ray energy spectrum into a plurality of channels, each of said channels corresponding to a particular energy within said spectrum;   computing a channel to energy conversion factor;   computing a stabilized lower boundary channel by multiplying the lower boundary energy value by the channel to energy conversion factor; and   computing a stabilized upper boundary channel by multiplying the upper boundary energy value by the channel to energy conversion factor.   
     
     
       17. The method as claimed in claim 16, wherein said step of computing said channel to energy conversion factor includes the steps of: determining a peak energy centroid channel; and   dividing said peak energy centroid channel by said peak energy value.   
     
     
       18. The method as claimed in claim 17, wherein said step of determining said peak energy centroid channel includes the steps of: determining a channel having a maximum number of counts within said stripping window;   determining a channel having approximately half the maximum number of counts;   determining a channel representing Compton background energy; and   computing the peak energy centroid channel from a relationship between the channel having approximately half the maximum number of counts and the channel representing the Compton background energy.